1. Field of the Invention
This invention relates to a load and torque sensing device. More particularly, this invention relates to a device for measuring the load and torque on an axle of a vehicle by measuring the resonant frequency of one or more wires, the tension on which is affected by the load.
2. Description of the Prior Art
Axle loads on vehicles, such as trucks, have previously been measured using various sensors, such as strain gauges. The use of such strain gauges in the sensing of axle loads are described in Kliever U.S. Pat. No. 3,488,997 and Videon U.S. Pat. No. 3,780,817.
Kliever U.S. Pat. No. 3,488,997 provides an apparatus for measuring the weight supported by landing gear of an aircraft by sensing the deflection of an arm rigidly mounted at an intermediate point to portions of the landing gear. The deflection at each end of the arm is sensed using a strain gauge, displacement sensor, force or movement transducer, etc.
Videon U.S. Pat. No. 3,780,817 teaches apparatus for measuring the weight on a truck axle using strain gauges which measure the shortening or flexing of a spring bar. The bar is connected at each end to the respective ends of an axle and acts to amplify the axle motion thereby subjecting the strain gauges to greater strains than if the gauges were mounted directly to the axle.
However, sensors using strain gauges suffer from several drawbacks including a rather large temperature sensitivity; as well as the small size and delicacy of the devices, which make then incompatible with the environment and skills associated with their use in truck axles. In addition, while the strain gauge itself is relatively inexpensive, the cost of a completed strain gauge sensor is not; due to the costs of installation, protection of the sensor, and connection to external equipment. Furthermore, the output signal from a strain gauge sensor is typically a low-level signal.
Other types of load sensing devices are known. For example, Gallo et al U.S. Pat. No. 4,274,501 describes a force measuring system wherein a load on a frame flexes an intermediate body on which are mounted two springs. Each spring is attached through a wire to an opposite end of a rigid measuring system. This system may comprise a force measuring cell with two vibrating strings.
Yamanaka U.S. Pat. No. 4,614,245 discloses a device for measuring a force utilizing two elastic cantilever arms between which is located a metal string. When the string is deflected, a permanent magnet located next to the string causes a current to be induced in the wire. The wire is coupled to an amplifier so that the induced currents set up a vibration in the string at its resonant frequency. The tension in the string can be calculated from this frequency.
Naito et al U.S. Pat. No. 4,623,813 describe a load sensor in which a shear-deformable member is located between a fixed member and a movable member. Flexible surface wave transducers comprising resonators are located on the shear-deformable member and have interdigited electrodes. Deformation of the shear-deformable member under load changes the clearance between the transducers which changes the resonant frequency of the resonators to change the oscillation frequency of the resonators.
In the measurement of vehicle loads, it is also known to measure the deflection of the vehicle springs using a sensor typically attached between the axle and the vehicle frame. However, such displacement sensors are typically not rugged and are mounted in a rather vulnerable location on the vehicle and are, therefore, easily damaged by dust, dirt, and grease. Sealing the sensor to prevent such damage increases the cost.
It would, therefore, be desirable to provide load sensing directly associated with the axle which could be housed within an axle housing, would be capable of operating accurately over a wide temperature range, would be rugged and long-lived, easily installed, inexpensive, and capable of providing a high-level electrical output. Advantageously, the sensor would be capable of independently measuring the load on each end of the axle and, in a preferred embodiment, be capable of measuring both axle load and torque.